1. Field of the Invention
The present invention relates to a semiconductor laser diode with a distributed feedback function.
2. Related Prior Art
A Japanese Patent application published as JP-H11-195836A has disclosed a semiconductor laser diode with the distributed feedback type (hereinafter denoted as DFB-LD) and a method for producing the laser diode. This process forms, at a first temperature, an optical guiding layer on a semiconductor substrate with a periodic undulation for the grating until the surface of the guiding layer becomes plane. Subsequently, as raising the temperature up to a second value, first and second spacer layers, both made of same material with the substrate, are formed thereon, and an active layer is finally grown at the second temperature. This method for the laser diode realizes reproducibility of the dimensions of the grating and good crystal quality of the active layer, which enables to produce the laser diode with superior performance and reliability by an enough yield.
In the DFB-LD with a type where the grating formed on the InP substrate is buried with AlGaInAs layer, a difference in the diffractive index between two layers both forming the grading may be expanded compared to a DFB-LD where the grating is buried by a GaInAsP layer. For example, when the band gap wavelength of the n-type AlGaInAs is 1.1 μm, the difference in the refractive index between the n-type InP and the n-type AlGaInAs may be 0.19, which is greater than the difference in the refractive index between the n-type GaInAsP and the n-type InP. That is, assuming the depth or the height of the periodic undulation for the grating is equivalent, the combination of semiconductor materials showing larger difference in the refractive index can give a larger coupling efficiency κL.
The coupling efficiency κL is preferably between 1.0 to 1.4 for the DFB-LD with the emission wavelength in the 1.3 μm bands and with the AlGaInAs layer for the grating. Forming the periodic undulation with a larger depth about 40 nm to get a large coupling efficiency κL, a thicker AlGaInAs layer becomes necessary to compensate the undulation to make the surface thereof flat. For instance, the thickness of the AlGaInAs layer becomes about 90 nm, which is a length from the bottom to the top of the undulation, to get a flat surface. Burying the grating with the undulation thereof about 40 nm by the AlGaInAs layer, a distance between the grating and the active layer becomes long because the AlGaInAs layer is necessary to be enough thick to get a flat surface, which prevents to increase the coupling efficiency κL and sometimes causes a failure in the side mode suppression ratio (SMSR).
The present invention is to provide a solution for the DFB-LD where the coupling efficiency becomes relatively large.